1. Field of the Invention
This invention relates to an electronic circuit device, specifically to an electronic circuit device having a structure to provide a power-supply capable of supporting fast signals in and above GHz band.
2. Description of the Related Art
A transistor which processes digital signals (and analog signals) in and above the GHz band makes fast switching with less than 100 ps of rise time and fall time. Electric energy corresponding to such fast switching has to be provided from the power-supply.
Issues of the power-supply are plainly explained through a metaphor of municipal water supply system. Even when a valve at a facet is opened very quickly, water does not come out of the faucet immediately. For example, transfer velocity of water power is 1500 m/s, thus it takes 6.7 μs for the water to run from the valve to the faucet when length between them is 10 mm. Even if the valve is fully opened in less than 6.7 μs, water only comes out from the faucet 6.7 μs later. In addition to that, it takes some time to accelerate the water to the velocity of 1500 m/s due to inertia of still water, the water does not reach such a high velocity because of friction resistance from a wall of the pipe due to viscosity of water. Water pressure in a main pipe connected with branch pipes to individual residential units is reduced to due to water discharged from the faucet since it is not possible to respond the rapid change when there is the inertia.
However, if the main pipe is 100 time larger than the branch pipe, the reduction in the water pressure is only 1/100 and practically negligible. Similar approach has been taken in conventional power-supply system. That is, a plane ground and a plane power-supply facing each other are provided. When they are not available, a bypass capacitor is used instead.
Transfer velocity of electric current due to charge transfer is equal to the velocity of an electromagnetic wave in a certain structure. When dielectric constant of the structure is 4, the transfer velocity is 1.5×108 m/s. When the electric current begins to flow in the structure, a voltage drop Vdrop expressed by the following equation is caused by a parasitic inductance Ls in a circuit.
 Vdrop=Lsdi/dt
A notion of the friction resistance from the wall of the water pipe is somewhat different in the world of electricity. In rapidly changing electric current, the corresponding notion to it is a characteristic impedance of a wiring. A direct current resistance related to a cross-section of the wiring is less than a hundredth of the characteristic impedance and gives no problem in general. However, the direct current resistance due to the skin effect caused by the rapid change has to be taken into calculation in the GHz band.
As appreciated from the foregoing, the power-supply to a fast switch should have no inertia, i.e. no parasitic inductance Ls and should have current capacity large enough to provide the switch with enough current when the switch is turned on. An electric current Imax corresponding to conductance G of the switch is expressed by:Imax=G Vdd
where Vdd is a power supply voltage. It is independent from load conditions since it is the maximum allowable electric current. A voltage at an end of a receiver is determined by the electric current and the load. Generally speaking, although the notion that a power-supply has to be supported according to the magnitude of the load is useful in slow switching, it should be designed based on the maximum electric current Imax in fast switching.
So, what would be a power-supply structure which can realize Ls=0 and Imax=G Vdd? The bypass capacitor always has the parasitic inductance Ls. The parasitic inductance Ls is approximately 200 pH with a 1005 type capacitor. Another parasitic inductance of approximately 200 pH from an electrode portion to connect the bypass capacitor is added to it. That is, the parasitic inductance is 400 pH in total. With a 0603 type capacitor on the other hand, corresponding parasitic inductances are 120 pH and 150 pH, respectively, resulting in a total parasitic inductance of 270 pH. It would be very difficult to reduce the total parasitic inductance including the inductance from the electrode portion to less than 100 pH, even if a smaller capacitance would be developed in future.
Next, the electric current I of the bypass capacitor system is 5 mA, when a conductance G of a bus driver is 0.02 S (Siemens) and Vdd is 1 V. Assuming that a pulse frequency is 10 GHz, a slew rate is about 30 ps. The voltage drop with the 0603 type capacitor is calculated by:Vdrop=270 pH×5 mA/30 ps=45 mV
Even when the minimum parasitic capacitance of 100 pH is assumed:Vdrop=100 pH×5 mA/30 ps=17 mV
When eight drivers are connected to the power-supply, the voltage drop for each case is 0.36 V and 0.136 V respectively, presenting a serious problem. Improving the power-supply with the bypass capacitor is very difficult as described above. Japanese Patent Application Publication No. 2001-210959 describes the issues described above.